Monitoring Control Utilization in an Industrial Facility, 1996 Process Control, Electrical & Info. Conference Proceedings

Today’s modem facilities put in large Distributed Control Systems (DKS’s) to monitor and control the unit operations. Typical DCS’s cost millions of dollars and cover thousands of control loops. The sad fact is that after the control system in installed and commissioned, many, if not most, of the control loops are not used the way the design engineer intended, and the control of the unit operations is not as good as it could be. While mills recognize the problem, it is a difficult one to solve. Operators do the best job they can, but they are not control engineers, nor are they expected to be able to tune control loops as a part of their normal job function. Thus, when a control loop goes unstable, or fails to perform like the operator expects, that control loop is switched from cascade to automatic, or from automatic to manual. If the operator is very contentious, he may make a note in the log book, or tell his foreman that he had a problem and took it off control. But more times than not, nothing happens to improve the control of the loop and fur the problem that caused the operator to take the loop off control. The next operator may put the loop back on automatic, just as a "test” to see if it works, or it may stay in manual for days. This is not an attempt to place blame, it’s just a report of the way things are in today’s plants where we have reduced staff sizes to the point that we don’t adequately cover all of the things we should, and many of these important functions are just ignored because they aren’t causing a “problem”. A recent survey conducted by Fadum Enterprises showed that only control loop tuning is not checked routinely by very many mills. Of those responding, 55% say they tune the control loops “Sometimes”, 8% tune them “Often”, and 38% tune them “On request only”. And while 49% of the mills have preventive maintenance programs for Measurements, only 10% have similar programs for control loop tuning. Some companies are beginning to realize that if it was worth investing millions to install these systems, it might be worth a few more thousands to monitor and improve the control of these processes.